1. Field of the Invention
This invention relates to a phase lock loop (PLL) device for a communication system, and in particular to a PLL device having frequency band selection and frequency band centering capabilities.
2. Discussion of the Related Art
Local oscillators are used in communications receivers for converting information signals to radio frequency (RF) and intermediate frequency (IF) signals. Conventional techniques for providing local oscillator (LO) signals use phase lock loop (PLL) designs including a voltage controlled oscillator (VCO) that are typically dedicated to generating LO signals at either a single frequency or over a narrow band of frequencies. Such narrow frequency bands are based on the frequency tuning range of varactor diodes within the VCO. Developments in communication technologies, such as the development in digital technology, have resulted in communication systems operating in widely separated frequency bands. For example, analog cellular systems (or AMPS, Advanced Mobil Phone Service) operate in frequency bands in the 800 MHz range, while personal communication service (PCS) systems operate in frequency bands in the 1900 MHz range. Thus, in communication devices that accommodate both AMPS and PCS systems, several LO signals in different and widely separate frequency bands are needed. Prior devices typically use a plurality of separate VCOs for generating LO signals falling in different frequency bands (e.g. U.S. Pat. No. 5,732,330 and 5,686,864). One prior device (U.S. Pat. No. 5,686,864) uses a variable VCO employing switchable parallel capacitors.
In conventional VCOs, the tunable frequency range is determined by the tunable capacitance range of the varactors in the VCO. The resonant frequency of the VCO circuit, however, is also influenced by the impedances of other circuit elements in the VCO. Thus, to ensure that the VCO can be tuned to desired frequencies, circuit elements in the VCO must be constructed to high precision (i.e. having small tolerances) or manually tuned so that the desired frequency is obtained while the varactor control voltage is as close as possible to the center of the tuning voltage range. This requires stringent manufacturing conditions, such as laser trimming, or post-adjustments of the circuit components after the circuits are constructed. These procedures are often costly and time-consuming. Further, these conditions require that the varactors have large capacitance tuning ranges to overcome tolerances in other circuit components. In addition, because the VCO may be required to lock to several different frequencies, at some of the frequencies, the varactors will be operating at control voltages that deviates from the center of their tuning voltage ranges. This again requires that the varactors have large tuning ranges. These problems are especially severe in multi-band VCOs since they involve additional circuit components that have to be built to high precision and post-adjusted.